Optical flip chip packages often include a substrate, a waveguide mounted on the substrate, and a flip chip die optically coupled to the waveguide. To achieve acceptable optical coupling between the optical flip chip die and the optical waveguide, the distance between the flip chip die and the substrate is controlled. If the distance between the flip chip die and the substrate is too large, optical coupling between the optical waveguide and the optical flip chip die may be less than optimal due to optical beam divergence. Conversely, if the distance between the flip chip die and the substrate is too small the optical waveguide and/or the optical flip chip die can be damaged during bonding of the chip to the substrate.
Methods of maintaining or increasing separation distance between the optical flip chip die and the substrate include using larger-than-normal solder balls to attach the optical flip chip die to the substrate. As optical flip chip packages exhibit increasingly finer pitch (i.e., as components become smaller and more densely packed together) and higher input/output density, solder bridging has started to occur. “Solder bridging” refers to electrical shorts created during soldering when solder melts and inadvertently connects adjacent or nearby electrical contacts. Increasing the amount of solder between the flip chip and the substrate increases the likelihood of solder bridging. Therefore, using larger solder balls to achieve a desired separation between the optical flip chip die and the substrate during the bonding process increases the likelihood of solder bridging.